Local Sidereal Time Calculator — LST for Any Date & Location
Local Sidereal Time (LST) tells you which part of the sky is on your meridian right now. Astronomers use it to determine which objects are currently observable. Enter a date, Universal Time and your longitude to compute LST and Julian Date.
hours
°
17h 57m 46.9s
- 1
Julian Date
2,460,847.5Continuous day count from noon UT on 1 Jan 4713 BC (standard Gregorian formula). - 2
Days from J2000.0
2,460,847.5 − 2451545 = 9,302.5 - 3
GMST (degrees)
(280.460618 + 360.985647 × 9,302.5) mod 360 = 269.4452 - 4
LST (degrees)
(269.4452 + 0) mod 360 = 269.4452 - 5
LST (hours)
269.4452 ÷ 15 = 17.9630
How does this calculator work?
LST = (GMST + λ) / 15 hours, where GMST (°) = 280.460618 + 360.985647 × D, D = Julian Date − 2451545.0 (days from J2000.0), and λ is your east longitude in degrees. A sidereal day is 23 h 56 m 4 s, so LST advances about 4 minutes per solar day. Enter UT (not local time) and east-positive longitude.
Formula
How this is calculated
Sidereal time tracks Earth's rotation relative to distant stars, not the Sun. A sidereal day is about 23 h 56 m 4 s — roughly 4 minutes shorter than a solar day — because Earth must rotate slightly more than one full turn to face the Sun again after orbiting it. That 4-minute daily gain means the star overhead at midnight tonight will be overhead at 11:56 pm one month from now.
The algorithm starts by computing the Julian Date (JD) from the Gregorian calendar date and Universal Time (UT). Days since the J2000.0 epoch (JD 2451545.0, 12:00 UT on 2000 January 1) are called D. Greenwich Mean Sidereal Time in degrees is then GMST = 280.46061837 + 360.98564736629 × D — the two-term approximation from Jean Meeus that is accurate to within a fraction of a second over decades. Local Sidereal Time is GMST plus the observer's east longitude, converted to the range 0°–360°, and divided by 15 to give hours.
This calculation uses the simplified two-term GMST series (accuracy < 0.1 s from 1950–2050) and ignores nutation and precession corrections needed for arcsecond astrometry. For telescope pointing or planetarium purposes the result is accurate enough; for professional astrometry, use a full SOFA implementation.
Frequently asked questions
An object's Right Ascension (RA) is fixed on the celestial sphere. It is on your meridian (highest in the sky, easiest to observe) when LST equals the object's RA. Knowing LST instantly tells you which objects are well-placed for observation right now.
Always enter Universal Time (UT/UTC), not local clock time. Convert by subtracting your UTC offset (e.g. for UTC+5:30 subtract 5.5 hours; for UTC−5 add 5 hours). The formula is defined in UT and gives wrong results with local time.
The Julian Date (JD) is a continuous count of days since noon UT on 1 January 4713 BC. It eliminates month/year ambiguity in time intervals, making arithmetic on dates trivial. J2000.0 corresponds to JD 2451545.0.
Also known as
TG we-Calculate Editorial Team. (2026). Local Sidereal Time Calculator — LST for Any Date & Location [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/sidereal-time-calculator
TG we-Calculate Editorial Team. "Local Sidereal Time Calculator — LST for Any Date & Location." TG we-Calculate. 2026. https://we-calculate.com/calculator/sidereal-time-calculator.
TG we-Calculate Editorial Team, "Local Sidereal Time Calculator — LST for Any Date & Location," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/sidereal-time-calculator
@misc{wecalculate_sidereal_time_calculator, title = {Local Sidereal Time Calculator — LST for Any Date & Location}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/sidereal-time-calculator}}, year = {2026}, note = {TG we-Calculate} }
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